研究了一种高精度的几何校正方法用于对锥束CT圆轨迹半覆盖成像进行几何校正。首先, 使用Otsu算法分割体模投影中钢球所在的区域, 并计算质心坐标。然后, 采用最小二乘算法对质心进行椭圆拟合, 并根据椭圆参数采用Cho的全覆盖几何校正算法计算探测器的旋转角度。最后, 顺时针旋转质心, 求得旋转的角度后, 再次进行椭圆拟合, 并根据得到的椭圆参数采用Noo的全覆盖几何校正算法计算除探测器倾斜角之外的所有几何参数。实验结果表明：探测器旋转角和偏转角的测量精度分别为0.02°和0.01°; 射线源到探测器和到旋转轴的距离的测量精度分别为0.05 mm和0.01 mm; 射线源在探测器上投影坐标的计算精度分别为0.07 mm和0.15 mm。由得到的结果可知, 所提出的校正方法有效地去除了几何伪影的干扰, 满足半覆盖成像图像重建的要求。

A high-precision method was investigated to realize the geometric calibration of half-cover scanning in circular cone-beam Computed Tomography (CT). First, projected areas of steel balls were segmented from the background by using the Otsu′s method, and the mass centers of balls were calculated. Then, the elliptic functions for mass centers were fitted by the least-square algorithm, and the rotation angle of detector was determined by Cho′s geometric calibration method for circular scanning. Finally, the rotated mass centers were fitted as two ellipses, and the rest geometric parameters except the tilted angle of detector were calculated by using Noo′s geometric calibration method for circular scanning. Experimental results indicate that the measuring precisions of rotation angle and skew angle for the detector are 0.02 °and 0.01°,respectively, the measuring precisions of source-detector-distance and source-object-distance are 0.05 mm and 0.01 mm, respectively; and the calculating precisions of orthogonal projection coordinates of source are 0.07 mm and 0.15 mm. The proposed method depress the geometric artifacts significantly, and can satisfy the demands of high-precision image reconstruction.